New electroencephalographic (EEG) and event-related potential (ERP) techniques have greatly improved spatial resolution in measures of brain electrical activity. There has, however, been relatively little application of these high-resolution EEG/ERP techniques in clinical research because of the time needed to apply dense arrays of electrodes, the lack of software to efficiently acquire and process the data and the cost of acquiring equipment. A Geodesic Sensor Net, developed by Electrical Geodesics Inc. (EGI), allows application of 128 electrodes in only 20 minutes. When used in conjunction with high-impedance Net amplifiers and advanced data processing software, this system yields "state of the art" spatiotemporal characterization of brain electrical fields. We are proposing a cost-effective Geodesic Sensor Net EEG System that will allow two well established electrophysiologic laboratories and other researchers doing clinical research to take advantage of the latest developments in high resolution EEG/ERP technology. The system includes two 128 channel EEG acquisition workstations and a central data processing workstation. These workstations will be located in a new High Resolution Electrophysiology Unit at New York State Psychiatric Institute (NYSPI) and will be linked together with a fast fiber optics network. The two laboratories and other users of the system represent a wide range of EEG/ERP research, including studies of depression in adolescents and adults, studies of schizophrenic and obsessive-compulsive disorders, studies in infants, developmental studies in children, and studies of normal aging, as well as Alzheimer's disease. The uniform and dense coverage of scalp afforded by the 128 channel EEG system and advanced software for spatiotemporal analysis of brain electrical fields will greatly enhance the topographic data in these studies. and will provide a basis for joint measurement in electrophysiologic and other imaging modalities, such as MR1.